THE European Space Agency has always lived in the shadow of its larger American cousin. But last week it stole the limelight when its Huygens probe gave us our first glimpse of the surface of Titan, Saturn鈥檚 giant moon. To be fair, it wasn鈥檛 all ESA鈥檚 doing, since Huygens hitched a lift on NASA鈥檚 Cassini spacecraft. But Huygens鈥檚 images are spectacular (see 鈥淥ne hour on Titan, forever bathed in glory鈥�). Perhaps the most surprising thing apart from the orange sky is that this alien moon looks strikingly familiar. And if the early signs are right, this is no accident: Titan shares an essential characteristic with Earth 鈥� it is wet.
On Earth, the liquid is water. It moulds the landscape and forms the weather. It is the defining substance. On Titan, where the surface temperature is -180 掳C, liquid methane may do the same job. Huygens has seen what looks like methane frost, fog and cloud layers. There are sinuous drainage systems, dead flat plains and rounded pebbles, all signs of flowing liquid. And the ground under Huygens is soft, quite possibly because it is wet.
For now the presence of liquid is educated guesswork. Although we have seen what look like lakes or seas bounded by shorelines, we have not detected liquid directly. That will have to wait for analysis of the data collected by Huygens, and from Cassini during its fly-by.
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If liquid does flow on Titan it will be remarkable. No other worlds in the solar system are dominated by liquid in this way. Venus has sulphuric acid rain, but it evaporates kilometres before it hits the ground. And although the Martian landscape was almost certainly shaped by water, most of the topography is an ancient fossil, whereas Titan鈥檚 surface seems fresh.
There is also an outside chance that the presence of liquid has implications for life. 杏吧原创s are now analysing gases in the atmosphere to look for the kind of chemistry that might have preceded life on Earth, but it is not impossible that some form of life exists on Titan. If so, it would probably need liquid to swim in. Our next visit to Titan could be truly revelatory.
